One of the final steps in the manufacture of magnetic recording disks used in fixed disk drives, after the disks have been coated with magnetic material, is to coat the disk with a lubricant, such as a perfluorinated polyether. The coating of lubricant protects the magnetic material and provides lubrication to minimize abrasion of the disk if a magnetic recording head accidentally or, on start up, contacts the disk. In such an event, the coating of lubricant substantially prevents loss of magnetic material (and the data stored therein) and minimizes friction between the head and the disk.
For recording and reading reliability, it is essential that the thickness of the coating of lubricant be very small so as not to increase substantially the distance between the head and the magnetic material. It is also essential that the thickness of the coating of lubricant be very uniform. Attempts have been made to achieve these goals by using conventional equipment which includes a processing tank which holds multiple disks in place as a liquid is pumped into and fills the processing tank to coat the disks with the lubricant. Typically, the liquid is discharged from the processing tank either by being pumped out or forced out under external pressure. Such action causes turbulence in the liquid and other detrimental effects which cause the coating of lubricant to be non-uniform across the disks. Other attempts to solve this problem involve placing the disks in the tank, filling the tank with a liquid including the lubricant and then gradually withdrawing the disks from the tank at a uniform rate. Any jerkiness in the withdrawal mechanism causes non-uniformity in the deposited coating.
The coating apparatus of this invention overcomes the shortcomings of the prior art approaches. This apparatus includes a processing tank and a supply tank below the processing tank for supplying a coating liquid to the processing tank for coating a plurality of substrates placed vertically in the processing tank for the duration of the coating process. After the substrates are placed in the processing tank, the processing tank is filled with the coating liquid. For coating magnetic disks, the coating liquid is a solution of a lubricant, such as a perfluorinated polyether, in a solvent, such as a chlorofluorocarbon, for example, Freon.TM.. Other applications may use a solution of a different coating material in a suitable solvent. In addition, the apparatus can be used with other forms of the coating liquid which may or may not include a solvent, such as a suspension or a slurry.
The coating of the disks occurs while the coating liquid drains from the processing tank by gravity, rather than being pumped out or forced out under pressure. During the draining process, none of the powered parts of the apparatus is operating, which eliminates vibrations that could cause waves on the coating liquid surface in the processing tank, which may lead to non-uniformity in the thickness of the lubricant coating. Coating liquid drains from the processing tank through an orifice in the bottom of the tank, through a metering valve and a pipe back into the supply tank.
The thickness of the lubricant coating is determined by the rate at which the coating liquid surface in the processing tank drops: the slower the drop rate, the thinner the coating. The coating thickness uniformity is determined by the uniformity of the rate at which the coating liquid surface in the processing tank drops. In a rectangular or cylindrical processing tank, that rate decreases as the processing tank empties, because the hydrostatic pressure, caused by the "head" of coating liquid (the vertical height of coating liquid in the tank), that drives the draining process decreases as the processing tank empties. Thus, a rectangular or cylindrical processing tank produces disks that have too thick a coating over the part of the disk that was uppermost in the processing tank, and too thin a coating over the part of the disk that was lowermost in the processing tank.
Prior art apparatus attempted to achieve a uniform coating thickness by inclining one of the side walls of the processing tank about 40.degree.-50.degree. to the vertical, and by giving the inclined wall a concave shape. This made the processing tank narrower towards the bottom of the tank, which reduced the surface area of the coating liquid towards the bottom of the tank. With this arrangement, the volume of coating liquid that was required to drain from the processing tank to decrease the level of the coating liquid surface in the tank by, for example, 1 mm, was smaller near the bottom of the tank than near the top of the tank. Reducing the surface area of the coating liquid in the tank towards the bottom of the tank compensated for the lower rate of coating liquid flow towards the bottom of the tank. This shape of the processing tank of the prior art provided an approximation to a constant rate of drop of the coating liquid surface in the tank. However, total coating uniformity was still a problem.
The processing tank of the prior art, with a concave inclined side wall, gave an acceptably uniform coating thickness only if the maximum total cross sectional area of the disks being coated was small compared with the cross sectional area of the processing tank at the same level of the coating liquid surface as that at which the disks (together with the fixture that held the disks in the processing tank) had their maximum cross sectional area. If this cross sectional area relationship was not small, the rate at which the level of the coating liquid surface dropped increased, and hence the coating thickness increased as the cross sectional area of the disks and fixture a the surface level of the coating liquid increased. To give an acceptably uniform coating thickness without using an excessively large processing tank, the inclined concave wall of the processing tank was given one or more protuberances coinciding with the vertical location of the disks and fixture, to compensate for the displacement of coating liquid caused by the disks and fixture, and to restore an acceptably constant rate of drop of the coating liquid surface level. Determining the optimum shape for the inclined wall of the processing tank of the prior art apparatus required considerable experimentation, and the result obtained was necessarily only an approximation to a truly uniform coating thickness.
For a number of reasons, it is desirable to improve the uniformity of the evaporative coating process. Rigid disks for digital data storage are constantly being made smaller, with a resulting increase in data density and reduction in bit area. The smaller bit size requires that a thinner coating of lubricant be used to prevent spacing losses from reducing the output from the record/reproduce head. With a thinner coating thickness, better uniformity is required to ensure that there are no uncoated areas on the disk. There are other applications for uniform evaporative coatings including the application of a soluble form of polytetrafluorethylene (PTFE), sold under the trademark Teflon.RTM. AF. The evaporative coating of the invention can be used to deposit on objects a very thin layer of PTFE having a uniformity that was impossible with previously known techniques. Evaporatively coating semiconductor wafers with a thin layer of PTFE of uniform thickness enables the PTFE layer to act as an etch resist in the manufacture of integrated circuits. Evaporatively coating a metal foil with a thin, uniform layer of PTFE allows the unique dielectric and insulating properties of PTFE to be exploited in capacitors, multilayer printed circuit boards and other electronic components. Evaporatively coating objects with a thin layer of PTFE enables the lubricating properties of PTFE to be exploited, for instance, the muzzle velocity and hence the range of ammunition can be substantially increased by a PTFE coating. PTFE coated ammunition also increases the service life of the barrels of guns firing such ammunition.
Many of the potential applications for evaporative coating involve objects having considerably more complex shapes than rigid magnetic storage disks. Moreover, evaporative coating processes use fluorocarbon and chlorofluorocarbon solvents, many of which are regarded as environmental hazards. The use of such solvents can be minimized (and hence the hazards of using such solvents reduced) by using processing tanks that are not substantially larger than the object or objects to be coated. Coating complex objects in tanks that are not significantly larger than the objects themselves make it difficult to obtain a uniform coating thickness.
To simplify the following explanation of the invention, the phrase "object to be coated" will refer to a single object or a plurality of objects that are to have a coating evaporatively deposited on them, together with the fixture (if any) used to support the object(s) in the processing tank. The term "cross sectional area" will mean a cross sectional area measured in the plane of the coating liquid surface in the processing tank, unless otherwise stated. The term "level of the coating liquid" means the distance of the coating liquid surface measured vertically from an arbitrary reference point. The term "every level of the coating liquid" means every level of the coating liquid at which the coating liquid surface contacts the object or objects that are part of the object to be coated.